How to add dimensions to responsive image/logo to increase Pagespeed Insights score?

Hi,

I have www.a-c-d.net. Pagespeed Insights writes to add dimensions to my logo. I already have this CSS:

.td-main-logo > img:nth-child(1) {
display: block;
margin-left: auto;
margin-right: auto;
width: 75%;
}

So I'm in doubt about how to add dimensions without breaking responsiveness?

Thanks

Ben